CHAPTER 4 - OIL SYSTEM
CONTENTS
PAGE
Typical Oil System ‘Wet’ Sump
02
Typical Oil System ‘Dry’ Sump
04
Oil Distribution
06
Main Bearings Locations
08
Main Bearing Lubrication
10
Oil Pump
12
Oil System – ‘Wet’ Sump System
Introduction
The oil then passes through a filter to ensure it is sufficiently
clean to prevent damage in the engine bearings and blockage
etc. then passes into the engine.
Lubrication is used between 2 sliding surfaces overcoming
friction, the resistance between sliding surfaces, and therefore
preventing or reducing component wear.
The oil pump are usually built into the internal structure of the
engine, and the filter, being a maintenance item, is attached to
the outside of the engine casing, where it is readily accessible.
The properties demanded from a lubricant vary considerably
with the particular application. A lubricant for a bicycle is not
suitable for a steam engine; the load and working temperature
vary in each instance. In general, a lubricating oil for any
engine should be of a suitable viscosity for maximum loads
and also retain this viscosity over the full range of engine
temperature.
Note: Viscosity = resistance to change of shape.
System Operation
In most piston engines, it is common practice for the oil to
stored in the sump, the bottom casing of the engine.
The oil us pulled out of the sump by an engine driven pump,
through a strainer. The oil provides the flow and the close
fitting components that it lubricates cause the pressure to rise.
When the engine and oil are cold, and at higher rpm’s when
warm, the pump could be damaged by the resultant excessive
pressures, This is prevented through a pressure relieve valve.
For instance, if the relief pressure is set at say 50psi, at 49psi
the valve would close and all the oil is delivered to the engine,
at 51psi the valve would open returning excess oil to the
sump, therefore limiting the pressure to 50psi.
2
‘WET’ SUMP SYSTEM
Pump
Engine Block
Filter
SUMP
Common for most vehicles
TYPICAL OIL SYSTEM - PISTON ENGINE
3
Oil System – ‘Dry’ Sump System
System Operation
In performance/rally cars and aircraft, the oil is held in a
separate tank as opposed to the engine sump. This gives
better protection against oil loss in cars; and helps to maintain
the correct level of lubrication in aircraft, particularly when
carrying out aerobatic manoeuvres.
The oil is taken out of the sump via a scavenge pump and
returned to the tank. The oil supply and distribution is the
same as previously described.
Because the oil is scavenged out of the sump, the system is
referred to a s the Dry Sump System.
4
‘DRY’ SUMP SYSTEM
Pump
Tank
Engine Block
Filter
Scavenge
Pump
SUMP
High Performance/Rally vehicles and Aircraft
TYPICAL OIL SYSTEM - PISTON ENGINE
5
Oil System – Distribution
Operation
Oil flowing from the pump is forced along internal
passageways, drilling and pipes to the areas needing
lubrication.
Oil escaping from the plain bearings ‘splash-lubricates’ the
ball and roller bearings, gear teeth, piston rings, cylinder walls
and other moving parts, and then drains to the sump or well at
the bottom of the crankcase.
More specific lubrication may be supplied by jets of oil
directed on to the particular contacting faces from holes drilled
in a nearby duct.
6
A detailed look at
bearing construction
and oil flows follows
TYPICAL OIL SYSTEM - PISTON ENGINE
7
Oil System – Bearing Locations
The illustration below shows the main bearing locations for: Crankshaft support bearings – five in this engine.
The Connecting rod to crankshaft bearings (the ‘big-ends’) –
one for each piston.
The gudgeon pins – one in each piston.
In addition (not illustrated) are other bearings required to
provide the smooth running of other engine components such
as the cam shaft, oil pump drive shaft, water pump drive shaft
etc.
8
9
Oil System – Bearing Lubrication
The illustration below shows a typical cross sectional view of a
plain bearing.
Maintenance
Regular servicing is necessary to prevent the build up of wear
material to the point where mechanical damage and/or
blockage preventing sufficient oil flow occurs. Both could
cause an increase in wear rate and therefore adversely affect
reliability.
The plain bearings are usually made up of two halves, called
‘shell’ bearings, and in the big-ends are fixed to prevent
rotation in the conrod.
The oil is fed to a distribution groove machined into the shells,
this allows the oil to travel all round the bearing.
The correct oil specification and scheduled maintenance
practices can be found in the Owners Handbook/Servicing
Book (vehicles) and the Maintenance Manual (aircraft).
The oil then spreads out from both sides of the distribution
groove, therefore providing an even thin film of oil between the
bearing and crankshaft surfaces.
Always change the oil and filter at the relevant engine life as
quoted in appropriate maintenance documentation.
The oil lubricating film is typically approximately .076mm thick
in a bearing diameter of 50mm.
Engine life is usually monitored as miles done in vehicles, and
hours run in aircraft.
The film works in 3 distinct layers, the outer layers clinging to
the separate surfaces and the central layer moving between
them.
Aircraft engine life could also be quoted as a combination of
hours and ‘cycles’: - one cycle being approximately equivalent
to a flight profile i.e. the different power levels selected during
one flight such as :-
System Indication
Maintaining oil pressure is critical to the engine health,
therefore it is indicated on the instrumented displays.
Start, taxi out, take-off, climb, cruise, descent, landing, taxi in,
shut down.
In most cars a simple indicator light is illuminated when
pressure drops below the critical minimum required to
maintain sufficient lubrication.
In performance cars and aircraft, usually a gauge is used to
display actual pressure, modern systems can also indicate a
warning when the minimum pressure is reached.
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Con Rod
Big End
Oil
Splash
Oil
Splash
Plain
Bearing
Lubricating
Oil Film
Oil Distribution
Groove
Internal
Oil
Feed
Crankshaft
Oil
Splash
Oil
Splash
TYPICAL OIL SYSTEM - PISTON ENGINE
11
Oil System – Internal Gear Type Pump
Description
This pump consists of two gears, one inside the other.
The smaller centre gear is the driving gear, driven by the
engine. The larger outer gear (or ‘idler’ gear) is driven by the
inner gear via the meshing of the gear teeth or ‘lobes’.
This pump is a positive displacement type and flow is relative
to pump rpm.
Operation
As the gears rotate the space between the inter-meshing
lobes increases pulling fluid in, then it decreases, pushing fluid
out.
The inlet and outlet ports in this pump are a similar shape to
kidneys, and therefore are known as kidney ports.
Fluid is pull in through the inlet kidney port and forced out
through the outlet kidney port.
General
The pump pictured below is mounted inside the engine sump
of a popular small car. It is obviously self lubricating and only
needs maintenance if oil pressure is low, especially at higher
speeds.
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Pump Assembly – (end plate removed)
Pump
Body
‘Kidney’ Ports
(shown dotted)
Driven
Gear
Driving
Gear
Yellow =
Inlet Flow
Red =
Outlet Flow
Rotating
Parts
TYPICAL OIL SYSTEM – Internal Gear Type Pump
13